OK there are several buttons you can press during POST to enter BIOS, and this all depends on the motherboard manufacturer or Chipset manufacturer.

The buttons that I have come accross in the past are Tab, F1, F4, and Del. The time to press these buttons is after the POST "beep" and just after the keyboard lights flash (Num Lock, Caps Lock and Scroll Lock)

Pressing them all at once will make no difference. If I don't know which button to press I cycle them all until I get into BIOS.